1. Field of the Invention
The invention relates to a composite active material that is able to reduce a reaction resistance as compared to an existing composite active material when mainly used in a lithium secondary battery, a manufacturing method for the composite active material, and a lithium secondary battery including the composite active material:
2. Description of Related Art
Secondary batteries are not only able to convert chemical energy to electric energy and to supply electric energy (be discharged) but also able to convert electric energy to chemical energy and to store chemical energy (be charged) by passing an electric current in a direction opposite to that during discharging. Among the secondary batteries, lithium secondary batteries have a high energy density, so the lithium secondary batteries are widely used as power supplies for mobile devices, such as laptop personal computers and cellular phones.
In the lithium secondary batteries, when graphite (denoted by C) is used as a negative electrode active material, the reaction expressed by the following formula (I) proceeds at a negative electrode during discharging.LixC6→6C+xLi++xe−  (I)(In the above-described formula (I), 0<x<1)Electrons produced from the reaction expressed by the above-described formula (I) pass through an external circuit, work at an external load and then reach a positive electrode. Lithium ions (Li+) produced from the above-described formula (I) move inside an electrolyte from the negative electrode side to the positive electrode side due to electroendosmosis. The electrolyte is held between the negative electrode and the positive electrode.
When lithium cobalt oxide (Li1−xCoO2) is used as a positive electrode active material, the reaction expressed by the following formula (II) proceeds at the positive electrode during discharging.Li1−xCoO2+xLi++xe−→LiCoO2  (II)(In the above-described formula (II), 0<x<1)During discharging, the reverse reactions of the above-described formula (I) and formula (II) respectively proceed at the negative electrode and the positive electrode, graphite (LixC6) in which lithium is intercalated by graphite intercalation is reproduced at the negative electrode, and lithium cobalt oxide (Li1−xCoO2) is reproduced at the positive electrode, so discharging becomes possible again.
Electrodes that are used in lithium secondary batteries are important members that determine the charge/discharge characteristics of the batteries, and various researches have been made so far. For example, Japanese Patent Application Publication No. 2010-073539 (JP 2010-073539 A) describes an electrode body that includes a positive electrode active material and a solid electrolyte. The positive electrode active material contains lithium cobalt oxide. A coating layer containing lithium niobate is formed on at least part of the surface of the lithium cobalt oxide. The solid electrolyte contains a solid sulfide.